1. Field of the Invention
The present invention relates to modulated and unmodualted material characterization techinques for characterizing materials and, more particularly, is directed towards a characterization module that has a unique configuration so that a variety of spectroscopy characterization techniques can be conveniently applied to material samples in a single apparatus. Using suitable material mounting adaptors, solids, liquids or even gases may be characterized.
2. Description of the Prior Art
Because of the recent advances in materials growth, for example with semiconductor materials and organic polymers, there is an ever increasing demand to utilize optical characterization techniques. These techniques include, but are not limited to photoreflectance, photomodulation, photoluminescence, thermoreflectance, thermomodulation, electroreflectance, electromodulation, piezoreflectance and piezmodulation. The position of optical transitions, as determined with the methods outlined above, provide information about alloy concentrations, doping or impurity concentrations, and epitaxial growth parameters.
The material characterization techniques mentioned above have been used for many years to determine material parametrs as described in the reference Handbook on Semiconductors, (T. S. Moss series ed., 1980) Vol. 2, "Optical Properties of Solids," North-Holland Publishing Company, 110-154, and references cited therein. For example, electroreflectance and photoreflectance spectroscopy has been used to evaluate surface states. In these techniques, the surface of a semiconductor is periodically excited either with an alternating electric field (electroreflectance) or an intensity modulated beam of light (photoreflectance). Changes in reflectivity in the sample due to this periodic excitation are then monitored. See U.S. Pat. No. 4,750,822 by Rosencwaig et al. which describes a method and apparatus for optically detecting surface states in materials.
However, it has become clear that more than one technique is often required to adequaely characterized the novel materials produced either by polymerization or epitaxial growth techniques. It has been shown that photoreflectance (a popular characterization method) could not be used to characterize an epitaxially grown quantum well sample but it could easily be characterized using piezoreflectance techniques in an article entitled, "Piezoreflectance as a Supplement to Photoreflectance for Nondestructive Characterization of GaAs/A1.sub.x Ga.sub.1-x As Multiple Quantum Wells," Tober et al., J. Appl. Phys., Vol. 64, No. 9, 1 Nov. 1988, pp. 4678-4682. Because of growth parameters piezoreflectance can be used to characterized double-barrier resonant tunneling devices but other optical techniques cannot. On the other hand, a class of epitaxially grown structures exists that, for one reason or another, can only be adequately characterized using a combination of complimentary modulation techniques as described in an article entitled "Piezoreflectance Characterization of Resonant Tunneling and Modulation-Doped Heterostructures", Tober et al., Journal of Electronic Materials, Vol. 18, No. 3, 1989, pp. 379-384. Polymers can also be characterized using modulated optical techniques as described in "Isomerization - Induced Evolution of Pieozreflectance Structures in Polyacetylene Films," Tober et al., Phys. Rev. B. Vol. 33, No. 12, 15 Jun. 1986, pp. 8768-8771.
Currently, there is no convenient single apparatus to test a particular material that can incorporate all the various modulation spectroscopy techniques. Each apparatus requires its own specific geometrical configuration and components (optical and electronic). Consequently, it is desirable to be able to have an apparatus that provides a simple means for performing these material characterization techniques in a single apparatus or module.